1. Field of the Invention
The present invention relates to locomotive compressor systems and, more particular, to a system for controlling locomotive compressors in a consist.
2. Description of the Related Art
In heavy haul freight train operations, there are frequently multiple locomotives at the head end of the train, all of which are providing tractive effort to move the train under lead control from the front-most locomotive. The locomotives are typically interconnected into multiple unit (MU) system by four air pipes, consisting of the brake pipe, 20-Pipe, 13-Pipe, and MR Pipe (main reservoir) and a standard “27 Pin” jumper cable. This combination allows the driver in the lead locomotive to drive the trailing locomotives as slaves with MU control of both propulsion and braking.
In an MU configuration, the main reservoirs on each of the locomotives are interconnected via the MR pipe end hose, making the combined MR volume available to the locomotive consist. Each locomotive also includes an air compressor that is used to pressurize the main reservoirs. In addition, the 27 pin train line includes a train line for MU compressor control (usually train line #22). This allows the compressor governor on the lead locomotive to simultaneously start and stop the compressors on all of the locomotives, resulting in very rapid filling of the interconnected MR system. In addition, the MU operation of the compressors assures uninterrupted, adequate air supply even if the compressor on the lead locomotive fails.
While the rapid filling of the MR system is desirable if all the MRs are at a low state of charge, or if the train brake system is discharged, because in these conditions the higher total air capacity of multiple compressors can be fully utilized. However, most of the time, the air system on the locomotives and train brakes are charged, and the air compressor is cycling between the compressor governor upper and lower control limits, typically between 120 psi and 140 psi. As a result, the full capacity of the compressors in the MU is generally not needed.
All of the air flow into the train brake pipe is controlled by the air brake system on the lead locomotive. The locomotive air brake system includes a nominally 19/64″ diameter choke restricting the flow between the outlet of MR2 and the inlet of the brake pipe pressure control circuit. Brake pipe pressure is typically fully charged at 90 psi. A full service brake pipe reduction is typically 26 psi, which corresponds to a 64 psi brake pipe pressure. To release the train brakes, the brake pipe is recharged to 90 psi. Because the brake pipe on the train is the length of the train, often in excess of 6000 feet, and due to effect of friction in the pipe, the brake pipe in the front of the train charges well before the brake pipe in the rear of the train. As a result, the brake pipe regulating device (brake pipe relay) in the locomotive brake system begins to throttle the air flow based on the brake pipe pressure at the head of the train before the brake pipe in the train is fully charged. The net combination of the low head pressure at recharge, which is 120 to 140 psi MR pressure flowing into a 64 to 90 psi brake pipe, the 19/64″ charging choke, and throttling of the brake pipe relay means that the rate of required air flow is much less than the air flow capacity of the compressor on just one locomotive.
In a MU consist, the combined air flow capacity from the compressors on each of the locomotives is thus much greater than required and, as a result, the compressor duty cycle is very short. For example, in some cases the MR recharge from 120 psi to 140 psi may take less than 30 seconds. This is undesirable for several reasons. First, the compressor start includes high inrush current, high accelerations, and high torque on the components, all of which are ultimately damaging to the compressor. Second, because the compressor runs for so short a time, it is not able to achieve optimum, stable operating temperature. As a result, there is an accelerated wear of cold parts due to transient thermal expansion issues and the cold compressor is more prone to accumulation of condensed water from the product air. Finally, in addition to issues of corrosion, the accumulation of liquid water can freeze in winter operation, thereby causing blockage of the compressor after cooler and discharge lines.
Preferably, the compressor has a longer duty cycle, so that the compressor and related components are heated due to the heat of compression to more or less the same temperature as the discharged air. The normal operating temperature of the compressor results in much less condensation in the compressor system, and enough heat in the after cooler and discharge lines to prevent any liquid water from freezing in those critical locations. Thus, while synchronous control of all the compressors in the locomotive consist might be an advantage during dry charge, or in the event of a failure of the compressor on the lead locomotive, synchronous control is clearly detrimental to compressor life and problematic during cold weather operation because the compressor duty cycle is too short.
In some circumstances a lead locomotive in a consist could be set up to allow for independent compressor control, so the pressure governor on each locomotive turns that compressor on and off independently. This control scheme addresses the issue of too much charge capacity because all the main reservoirs are connected by the MR pipe and therefore the MR pressure on each locomotive is nominally the same and because there is a natural tolerance in the pressure governor settings on each locomotive compressor control. However, in this scheme, one compressor in the locomotive consist will turn on at a higher pressure than the other compressors in the consist due to tolerance variations of the pressure governors and will provide all of the air for the train and, as a result, the compressor utilization and compressor maintenance demand is unbalanced. Typically compressor maintenance is done on a planned, periodic schedule, with certain maintenance actions occurring at regular calendar intervals. Thus, the compressors subject to this control scheme will have done more work during the maintenance interval than others, so some compressors will be maintained too late and some serviced earlier than needed.